Arenes

• Describe Kekulé's structure

  A C₆H₆ molecule with alternating single and double carbon bonds (a tri-ene)

  
• What evidence is there that Kekulé's structure is incorrect?

  • The lengths of the carbon bonds in benzene are all equal
  • Benzene does not decolourise when shaken with bromine water
  • The enthalpy change of hydrogenation of benzene is less exothermic than Kekulé's theoretical value
• What is the actual structure of benzene?


  Benzene has a pi system comprised of p-orbital electrons delocalised around all six carbons due to their sideways overlap (which is more energetically stable)
• Benzene undergoes electrophilic substitution reactions in four stages:

  1. The generation of an electrophile with a catalyst
  2. The electrophile attaches to the benzene, forming an intermediate ion
  3. The hydrogen ion is lost from the intermediate, reforming the delocalised ring
  4. The catalyst is regenerated
• What catalyst is used for the bromination of benzene?


  FeBr₃
• What catalyst is used for the alkylation of benzene?


  AlCl₃
• What is the catalyst for the acylation of benzene?


  AlCl₃
• Under what conditions does the acylation of benzene occur?


  Under reflux at 60°C for 30 minutes
• What is the functional group of an acylchloride?


  R-COCl

  
• What is the catalyst for the nitration of benzene?


  H₂SO₄
• Under what conditions does the nitration of benzene occur?


  At 50°C where the nitrating mixture (H₂SO₄ and HNO₃) is concentrated
• What is the equation for the formation of the catalyst in the nitration of benzene?


  HNO₃ + H₂SO₄ ➞ HSO₄⁻ + H₂NO₃⁺ H₂NO₃⁺ ➞ H₂O + NO₂⁺
• Why do reactions with benzene require a catalyst?


  Benzene has a lower electron density due to the electrons being delocalised around the ring rather than in a localised pi bond. So benzene induces a weaker dipole across an electrophile, therefore a catalyst is needed to sufficiently polarise the molecule
• Which positions do activating groups direct to?


  2 and 4
• Which position do deactivating groups direct to?


  3
• Is –OH activating or deactivating?


  Activating
• Is –NH₂ activating or deactivating?


  Activating
• Is –CH₃ activating or deactivating?


  Activating
• Is –NO₂ activating or deactivating?


  Deactivating
• Why is phenol sparingly soluble in water?

  Although the –OH group can form hydrogen bonds, the non-polar benzene ring can only form weak induced dipole forces which does not compensate for the hydrogen bonds broken between water molecules in order for phenol to be soluble
• Why is phenol a weak acid?


  Phenol only partially dissociates (into phenoxide and hydrogen ions)

  
• Which bases does phenol react with?


  Only strong bases (like NaOH) and not weak bases (like Na₂CO₃)
• What observations can be noted when phenol reacts with concentrated HCl?


  A white precipitate forms, because the equilibrium shifts towards the less soluble phenol so to reduce the number of hydrogen ions
• What observations can be noted in the bromination of phenol?

  The solution decolourises and then a white precipitate forms
• What conditions differ in the nitration of phenol compared to the nitration of benzene?


  In the nitration of phenol, the concentrated sulfuric acid catalyst is not required (nor the concentrated nitric acid) and it is carried out at room temperature
• Why is phenol more reactive than benzene?

  The –OH group activates the ring as the lone pair in the p-orbital of the oxygen align with the pi system, increasing the electron density in the ring which means phenol can induce a stronger dipole across an electrophile
• What type of reaction does benzene undergo?


  Electrophilic substitution
• What products form in the nitration of phenol?


  A mixture of 2-nitrophenol and 4-nitrophenol
• Why are –OH and –NH₂ activating groups?


  Because they donate a lone pair of p-orbital electrons to the pi system which increases the electron density so the molecule can more strongly induce a dipole
• Why is –NO₂ a deactivating group?


  Because it withdraws electrons from the pi system, due to being an electrophile